Thermoset molding compositions known in the art are generally thermosetting resins containing inorganic fillers and/or fibers. Upon heating, thermoset monomers initially exhibit viscosities low enough to allow for melt processing and molding of an article from the filled monomer composition. Upon further heating, the thermosetting monomers react and cure to form hard resins with high stiffness.
One industrial use of thermoset compositions is the molding of automotive body panels. These panels preferably exhibit high dimensional stability and a smooth as-molded surface. It is also preferred that the dimensions of the molded parts conform closely to those of the molds used to prepare them.
Thermoset compositions based on unsaturated polyester resins and styrene are known to exhibit reduced shrinkage and improved surface properties when they incorporate a so-called low-profile additive, such as a polymethacrylate copolymer. See, for example, V. A. Pattison et al. J. Appl Poly. Sci, volume 18, pages 2763-2771 (1974). Although known low-profile additives improve the performance of the polyester thermosets, there is a need for compositions exhibiting further improvements, particularly in surface characteristics.
U.S. Pat. No. 6,352,782 to Yeager et al. describes thermoset compositions comprising poly(arylene ether) resins that have been capped with ethylenically unsaturated groups. These compositions exhibit desirable properties including high glass transition temperatures and low coefficients of thermal expansion. However, low-profile additives known for polyester thermosets are ineffective in the poly (arylene ether)-containing compositions.
There remains a need for thermoset compositions exhibiting reduced shrinkage on molding and improved surface characteristics.